Many applications (e.g., motor drive systems for electric or hybrid motor vehicles) utilize a voltage source inverter (VSI). Typically, the VSI supplies voltage to a multi-phase electric motor (e.g., an alternating current (AC) motor), which in turn, drives the wheels of the electric or hybrid motor vehicle.
Thermal management issues concerning power switches within the VSI arise when the VSI supplies voltage to the electric motor when the electric motor is producing large amounts of torque at low output frequencies (including zero output frequency). That is, the VSI is applying a low (or zero) voltage across the motor phases while receiving a constant input voltage (i.e., a direct current (DC) voltage). When operating at low output frequencies, the average voltage supplied by the VSI is small compared to the input voltage; however, the current the VSI supplies may be at the switch-rated current for the VSI.
These thermal management issues typically arise when the electric motor is producing large amounts of torque at low or zero output frequencies because of slowly changing AC currents in the VSI. That is, a power switch in the VSI may be required to continuously carry the peak phase current or at least carry the peak phase current for an extended period of time, which causes constant high power dissipation inside the power switch and represents a worst-case operating condition for a VSI switch.
A conventional solution for this situation is to time-limit the VSI operation at zero output frequency and full output current. Unfortunately, this solution is problematic for some applications (e.g., electric and hybrid motor vehicles) because it limits the time during which large amounts of torque may be applied by the electric motor (e.g., during vehicle start).
Accordingly, it is desirable to apparatus, systems, and methods for reducing inverter losses when the electric motor is producing large amounts of torque at low output frequencies (including a zero output frequency). Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.